CultCache stages a temp file and renames it over the store; the temp file is
created under Idunn's UMask=027, so the visible projection was 0640 until
publish_projection_mode chmodded it after the rename and the directory fsync.
A reader running as another user (Odin) failed with EACCES in that window.
The store write now runs on a helper thread that unshares CLONE_FS and sets
umask 022, so the staged file and the .lock are created 0644 and never visible
in another mode; other threads keep 027. publish_projection_mode is deleted.
CultCache has no mode parameter, so this lives in Idunn. The ACL note in
migration.md now says the ACL is no longer required.
Test: a reader polls the directory across 300 publishes and records the mode of
every store and lock inode it sees; all must be 0644.
Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>
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